Yes, there was a lot of other stuff to worry about in that call, at least for short-term investors:

Facebook's revenue and earnings performance was merely in line with guidance that most observers assumed had been low-balled

Facebook will spend a lot more money this year than investors thought, which will hammer the company's profit margins

The mobile transition accelerated in Q2, with many Facebook users now using the site only via their mobile devices (Facebook makes a lot less money from mobile users than it does from desktop users)

The company's critical new ad product, "Sponsored Stories," had gotten off to promising start, but it still accounted for a relatively small amount of revenue (relatively) and did not seem likely to offset the maturation/stagnation of the core ad business

Most alarmingly, the number of ad units that the company served in its most important market, the U.S., declined 2% year over year--an unheard-of event for a company that most investors viewed as a "hyper-growth" company

I got all that. And, combining that news with 1) the huge lock-up releases that are coming in the next few months (allowing hundreds of millions of additional Facebook shares to be sold), as well as 2) the increasing likelihood (in my opinion) that Facebook will not just fail to exceed but, in fact, will miss Wall Street's current expectations (more on this soon), it wasn't surprising to see the stock sell off hard.

What I didn't focus on until now, though, was something else the company said on the call:

Facebook's web-based users in the U.S. declined year over year, and the company's core U.S. business may be shrinking.

Ad impressions continued the recent trend of growing more slowly than users as more of our usage is on mobile devices. This trend is particularly true in markets such as the U.S., where smartphone use is expanding rapidly. The overall number of ads delivered in the U.S. this quarter decreased 2% year-over-year despite a 10% increase in daily users and despite the increase in ads per page from the product changes I mentioned earlier, as daily Web users in the U.S. declined in favor of mobile users. And we're seeing similar trends in other developed markets.

So, there it is: The number of web-based users in the U.S. and other developed markets is shrinking.

Worse, the core business in the U.S. may be shrinking.

How do we know that?

By combining the information above with some more information from David Ebersman's remarks:

The 9% increase in price per ad [global] was driven primarily by the United States, where CPMs [price per thousand ads] increased by over 20% due in large part to the ramp-up of Sponsored Stories in News Feed on both PCs and mobile devices.

So, the vast majority of the impressive 20% per-ad price increase in the U.S. was driven by the rollout of a new product, "Sponsored Stories," which commands much higher prices and is delivered to all users, not just web users. Facebook's North American ad revenue grew only 22% in the quarter. So that suggests that, excluding the impact of the new product, the core U.S. business is flat or shrinking (we don't know how much of growth came from Canada).

Critically, almost all of Facebook's revenue still comes from web-based users. The US and Canada still account for 48% of Facebook's revenue, and the U.S., Canada, and Europe (where there are many more developed markets) account for 78% of revenue. So Facebook's current business is very exposed to this trend.

Again, Facebook's new "Sponsored Stories" product drove a 20% increase in per-ad pricing in the U.S. despite the ad-unit and user decline. That's important and encouraging. But despite this vast improvement in pricing, ad revenue in the U.S. and Canada only rose 22% year over year.

Although Facebook will continue to get a pricing boost as it rolls out more "Sponsored Stories," if user growth flattens or declines, the impact these increases have on Facebook's revenue growth will be limited.

Unless or until Facebook rolls out entirely new products, therefore, the key driver of the company's growth will be the ongoing rollout of "Sponsored Stories." Right now, at least in the developed markets, that seems to be the only Facebook revenue component that is growing.

No one outside the company knows yet how big a product "Sponsored Stories" can be. This quarter, the third quarter, will be the first quarter in which the product will have been available on mobile in the entire quarter. As of the conference call in mid-July, "Sponsored Stories" were generating annualized revenue of about $400 million a year ("$1 million a day"). That's a nice chunk of revenue, but it won't move the needle much on a $5 billion revenue base.

So the key question is how fully penetrated the "Sponsored Stories" opportunity is. If it's only, say, 5%, the opportunity is potentially huge. If it's 25%-50% in the U.S., meanwhile, "Sponsored Stories" won't make a radical difference, especially if Facebook's core web-based ad business continues to shrink.